Powder molding method of graphite bipolar plate and graphite composite bipolar plate

Abstract

The application discloses a powder moulding forming method of graphite bipolar plates and a graphite composite bipolar plate, and belongs to the technical field of graphite bipolar plate manufacturing. The method comprises the following steps: an electric field is applied to the bipolar plate blank during the moulding forming process, the applied electric field is used to provide a moment, so that the graphite sheet layers are arranged in the direction perpendicular to the plate surface during the solidification forming process; and the electric field intensity, the moulding pressure and the temperature are kept synchronous increasing changes in a certain time period during the forming process. The purpose of the application is to solve the balance problem between the conductivity and the mechanical strength of the final product in the existing powder moulding forming bipolar plate, more resin materials can be used on the basis of meeting the same conductivity, so that the mechanical strength of the bipolar plate is improved; or in the case of the same mechanical strength, the obtained bipolar plate can obtain higher conductivity.

Description

Technical Field

[0001] This invention relates to the field of graphite bipolar plate manufacturing technology, and in particular to a powder molding method for graphite bipolar plates and a graphite composite bipolar plate. Background Technology

[0002] The core contradiction in the performance of powder-molded graphite composite bipolar plates lies in the fact that while high graphite content is beneficial for conductivity, it leads to decreased strength and increased brittleness; conversely, while low graphite content ensures mechanical strength, it reduces conductivity. Therefore, the ratio of graphite to other components in graphite composite bipolar plates must strike a balance between conductivity and mechanical strength.

[0003] In bipolar plates manufactured using traditional molding processes, the orientation of the graphite sheets inside is random. Graphite sheets themselves exhibit significant anisotropy: their in-plane conductivity is extremely high, while their interlayer conductivity is relatively low. Therefore, the random orientation of the graphite sheets results in poor conductivity in the final bipolar plate. Consequently, to manufacture a bipolar plate with satisfactory conductivity, the required graphite material ratio cannot be lower than a certain value, thus preventing further enhancement of the bipolar plate's mechanical strength.

[0004] In view of this, a new technical solution is needed to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to solve the problem of balancing conductivity and mechanical strength in existing powder-molded bipolar plates. Specifically, existing technologies excessively increase the proportion of graphite to meet conductivity requirements, resulting in decreased strength and increased brittleness of the final bipolar plate.

[0006] Therefore, the first aspect of the present invention provides a powder molding method for graphite bipolar plates, which includes the following steps: An electric field is applied to the bipolar plate blank during the molding process. The applied electric field is used to provide a torque so that the graphite sheets align in a direction perpendicular to the surface of the bipolar plate during the curing process. Furthermore, during a certain period of the molding process, the electric field strength, molding pressure, and temperature all increase synchronously.

[0007] As a further improvement, the electric field strength is gradually increased from 0.5 kV / mm to 3 kV / mm, the molding pressure is gradually increased from 5 MPa to 30 MPa, and the temperature is gradually increased from 50°C to 130°C for 10 minutes.

[0008] As a further improvement, an initial pressurization step is included before the step of applying the electric field: Apply a first pressure to the bipolar plate blank to expel air from the blank and reduce its porosity.

[0009] As a further improvement, the first pressure is 5 MPa for 2 minutes.

[0010] As a further improvement, a finalization step is included after the synchronous incremental step: Maintain the electric field strength, molding pressure, and temperature at a fixed value for a period of time.

[0011] As a further improvement, the molding pressure is maintained at 30 MPa, the temperature is maintained at 150°C, the electric field strength is 3.0 kV / mm, and the duration is 3 minutes.

[0012] As a further improvement, a curing step is included after the shaping step: Remove the electric field and maintain the pressure and temperature for a period of time.

[0013] As a further improvement, the molding pressure is maintained at 30 MPa, and the temperature is increased to 220°C for 5 minutes.

[0014] As a further improvement, after the curing step, a demolding and cleaning step is also included.

[0015] A second aspect of the present invention provides a graphite composite bipolar plate prepared by the above method.

[0016] As a further improvement, the mass ratio of graphite to resin binder in the graphite composite bipolar plate is 7:3 to 8:2.

[0017] Compared with existing technologies, the present invention brings the following technical effects: This invention applies an electric field to the bipolar plate blank during the molding process. This applied electric field provides a torque, causing the graphite sheets to align perpendicular to the surface of the bipolar plate during curing. Furthermore, the electric field strength, pressure, and temperature gradually increase over a certain period during the molding process. This setup maximizes the guiding effect of the electric field on the graphite sheets: initially, the powder has high porosity, is filled with air, and has low dielectric strength, making it easily broken down under high pressure; therefore, the electric field strength also increases from low to high. As the pressure gradually increases, the powder density increases, and the dielectric strength of the blank also increases. Therefore, simultaneously increasing the pressure and electric field strength ensures that the electric field remains within the tolerance range of the current density. Within a safe range, high-voltage electric field breakdown is avoided, preventing damage to the mold and product. As the temperature gradually increases, the resin material gradually solidifies and eventually sets. During this process, the electric field increases with temperature, ensuring that the electric field effectively guides the graphite sheets. The final result is that most of the graphite sheets inside the bipolar plate are positioned or tend to be positioned perpendicular to the plate surface. This micro-arrangement fully utilizes the high conductivity of the graphite sheets, significantly improving the interlayer conductivity of the bipolar plate and thus reducing the surface resistance. Using this method to produce bipolar plates, the proportion of resin material can be further increased while ensuring that the surface resistance of the bipolar plate meets the requirements, thereby improving the mechanical strength of the final product. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 The diagram illustrates the changes in electric field, pressure, and temperature in the powder molding method of a graphite composite bipolar plate according to the present invention. Figure 2 The image shows a scanning electron microscope (SEM) image of a conventional molded product. Figure 3 A scanning electron microscope image of the molded product of the present invention is shown. Detailed Implementation

[0020] The embodiments of the present invention are described in detail below. The described embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0021] Example This embodiment provides a powder molding method for graphite composite bipolar plates and the bipolar plates obtained by this method.

[0022] Please see Figure 1 Specifically, the powder compression molding method includes the following steps: S100. Prepare the mold and perform pretreatment: The mold includes an upper mold and a lower mold. The pressing surfaces of the upper and lower molds form a cavity when the mold is closed. The shape and size of the cavity are consistent with the final product bipolar plate. In this invention, the upper mold and the lower mold are respectively connected to two electrodes, one of which is grounded and the other is connected to a high-voltage power supply. Since the two molds are aligned vertically, a highly uniform vertical electric field is generated between the two molds.

[0023] It is important to note that the mold frame must be made of high-strength insulating material, such as insulating ceramic, to ensure complete isolation between the two templates and the mold frame, preventing electric field leakage and short circuits. The mold itself includes temperature and pressure regulation structures and control components, which will not be elaborated here.

[0024] S200. Prepare graphite composite bipolar plate blanks: Mix graphite and phenolic resin evenly at a mass ratio of 8:2, and pre-press into blanks.

[0025] The order of steps S100 and S200 above can be reversed.

[0026] S300, molding is carried out, which is divided into three stages: S301, Initial pressurization stage: Control the mold to close and apply a low pressure of 5MPa to initially compact the blank and expel most of the air to reduce porosity. This stage lasts for 2 minutes.

[0027] S302, Synchronous Incremental Stage: The pressure slowly increases from 5MPa to 30MPa, the electric field strength gradually increases from 0.5 kV / mm to 3kV / mm, and the temperature increases from 50℃ to 130℃. This stage lasts for 10 minutes.

[0028] In the initial stage of molding, the preform powder still contains pores filled with air, resulting in low dielectric strength. It is easily broken down under high pressure, so the initial applied electric field strength cannot be too high, otherwise breakdown will occur. As the pressure gradually increases, the powder density increases, and its dielectric strength also improves. The voltage the preform can withstand also increases. Therefore, simultaneously increasing the pressure and electric field strength ensures that the electric field remains within the safe range that the current density can withstand, thus avoiding high-voltage electric field breakdown and damage to the mold and product. Furthermore, due to the characteristics of phenolic resin, it gradually softens as the temperature rises (but before reaching the curing temperature), and the overall viscosity of the preform tends to decrease. This change undoubtedly reduces the resistance to the graphite flakes' orientation changes, which is beneficial for the electric field to guide the graphite flakes.

[0029] S303, Shaping Stage: Pressure 30MPa, overall temperature maintained at 150℃ (below the curing temperature of phenolic resin), electric field strength 3.0 kV / mm, pressure held for 3 minutes.

[0030] Principle introduction: The resin softening temperature is 120℃ and the pre-curing temperature is 160℃. Before reaching this temperature, the viscosity of the system is not high, and the graphite sheets are most easily oriented under the action of electric field torque. Before the resin cross-links and the viscosity begins to increase, a strong electric field is maintained to lock the oriented structure.

[0031] S304, Curing Stage: Remove the electric field, raise the overall temperature of the mold to 220℃ (the complete curing temperature of phenolic resin), maintain the pressure at 30MPa, and hold the pressure for 5 minutes.

[0032] After step S303, the resin cross-linking network is initially formed, and the orientation structure of the graphite sheets in the preform is basically finalized. Then the electric field is removed, and the temperature and pressure are continued to complete the final curing.

[0033] S400, Demolding and Cleaning. Demold the product and perform surface treatment.

[0034] Please see Figure 2 and Figure 3 , Figure 2 These are scanning electron microscope images of traditional molded products. Figure 3 These are scanning electron microscope (SEM) images of the molded product of this application. A comparison of the two images reveals that the molded product of this application exhibits a large number of vertically oriented structures (i.e., perpendicular to the bipolar plate surface) at the microscopic level. This microstructure is the result of the ordered orientation of graphite sheets under a strong electric field, which can improve the interlayer conductivity of the bipolar plate. Simultaneously, the vertically oriented microstructure can also enhance the gas barrier properties within the bipolar plate, reducing its brittleness. In contrast, traditional molded products have significant separation between layers, which is a major reason for the high surface resistivity of the final product.

[0035] The conductivity of bipolar plates obtained by different molding methods under the same conditions of component ratio and other parameters is shown in the table below: As shown in the table above, the orientation of the graphite sheets inside the bipolar plate obtained by the traditional molding method is random. Graphite sheets themselves exhibit significant anisotropy: their in-plane conductivity is extremely high, while their interlayer conductivity is relatively low. Therefore, the randomly oriented graphite sheets result in poor final conductivity of the bipolar plate, with an average conductivity of approximately 300 S / cm.

[0036] The bipolar plate obtained by the molding method proposed in this application has graphite sheets oriented in an orderly manner under the action of a strong electric field (of course, it must be combined with the synergistic effect of pressure and temperature). Finally, by utilizing the high conductivity of the graphite sheets, the conductivity of the final product is improved. The average measured conductivity is about 450 S / cm, which is 50% higher than that of traditional products.

[0037] This application also sets up a ratio, that is, applying an electric field after the traditional molding is completed. Although this method can change the orientation of the graphite sheets in the bipolar plate, the effect is generally poor, with an average measured conductivity of about 320 S / cm. The reason for this is that the phenolic resin has already cured in the later stage of molding. At this time, the resistance to the orientation change of the graphite sheets is too great when an electric field is applied, so the effect is not significant.

[0038] In summary, this application utilizes the characteristic of graphite sheets as electric dipoles to apply an electric field to the bipolar plate blank during the molding process. This applied electric field provides a torque, causing the graphite sheets to align perpendicular to the surface of the bipolar plate during curing. Furthermore, the electric field strength, pressure, and temperature gradually increase over a certain period during the molding process. This arrangement maximizes the guiding effect of the electric field on the graphite sheets: initially, the powder has high porosity, is filled with air, and has low dielectric strength, making it easily broken down under high pressure; therefore, the electric field strength also increases from low to high. As the pressure gradually increases, the powder density increases, and the dielectric strength of the blank also increases. Therefore, simultaneously increasing the pressure and electric field strength ensures the electric field... By maintaining a safe density within acceptable limits, high-voltage electric field breakdown can be avoided, preventing damage to the mold and product. As the temperature gradually increases, the resin material gradually solidifies and sets. During this process, the electric field increases with temperature, ensuring effective guidance of the graphite sheets. The final result is that the vast majority of graphite sheets within the bipolar plate are positioned or tend to be positioned perpendicular to the plate surface. This microstructure fully utilizes the high conductivity within the graphite sheets, significantly improving the interlayer conductivity of the bipolar plate and thus reducing its surface resistance. Using this method to produce bipolar plates allows for a higher proportion of resin material while ensuring the surface resistance meets requirements, thereby increasing the mechanical strength of the final product. Typically, graphite accounts for 80%–90% of the raw material, with the remainder being unrefined material. However, using the process of this invention, the graphite material proportion can be reduced to 70%, leaving more space for resin material while ensuring that the conductivity of the finished bipolar plate does not decrease.

[0039] It should be noted that electricity and magnetism are interconnected and inseparable. The above embodiments only use an electric field as an example to illustrate the essence of the invention. Those skilled in the art will readily realize that the properties of graphite sheets as magnetic dipoles can be utilized by applying an external magnetic field to achieve the same effect of ordered orientation of the graphite sheets. All such solutions that do not depart from the spirit of the invention should fall within the protection scope of the invention.

[0040] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A powder molding method for graphite bipolar plates, characterized in that, Includes the following steps: Graphite and phenolic resin are mixed evenly according to a preset ratio and then pre-pressed into a blank. An electric field is applied to the bipolar plate blank during the molding process. The applied electric field is used to provide a torque so that the graphite sheets align in a direction perpendicular to the surface of the bipolar plate during the curing process. Furthermore, during a certain period of the molding process, the electric field strength, molding pressure, and temperature all increase synchronously. At this time, the electric field strength gradually increases from 0.5 kV / mm to 3 kV / mm, the molding pressure gradually increases from 5 MPa to 30 MPa, and the temperature gradually increases from 50℃ to 130℃ for 10 minutes. The initial pressurization step is included before the step of applying the electric field: Apply a first pressure to the bipolar plate blank to expel air from the blank and reduce its porosity; Following the synchronous incremental steps, a finalization step is also included: The electric field strength, molding pressure, and temperature are kept at a fixed value for a period of time. The molding pressure is maintained at 30 MPa, the temperature is maintained at 150°C, and the electric field strength is 3.0 kV / mm for 3 minutes.

2. The method as described in claim 1, characterized in that, The first pressure is 5 MPa, and the duration is 2 minutes.

3. The method as described in claim 1, characterized in that, Following the shaping step, a curing step is also included: Remove the electric field and maintain the pressure and temperature for a period of time.

4. The method as described in claim 3, characterized in that, The molding pressure is maintained at 30 MPa, and the temperature is raised to 220°C for 5 minutes.

5. A graphite bipolar plate prepared by the method described in any one of claims 1-4.

6. The graphite bipolar plate as described in claim 5, characterized in that, The mass ratio of graphite to phenolic resin in the graphite composite bipolar plate is 7:3 to 8:2.

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